The present invention is directed to a method and apparatus for applying a uniform film of fluid to a flat surface being conveyed through a workstation and specifically for applying a uniform film of developer to lithographic printing plates in a developing station. The invention is particularly directed to the use of a wire-wound rod for uniformly distributing the fluid over the workpiece and to the technique for metering and delivering the fluid to the wire-wound rod. The present invention is applicable to various planar workpieces and various types of workstations but is particularly applicable to lithographic printing plates which have been imaged and require the application of a developer to remove the areas of the coating on the plate which have been rendered soluble by the imaging process. Although the invention has a broader application, it will be described with particular reference to lithographic printing plate development.
One method for the development of the imaged plates entails the application of a thin film or layer of developing solution to the imaged plate surface of each imaged plate to be developed. This thin film of developer solution is allowed to dwell on the plate for a time sufficient to complete the development and then rinsed from the plate. Because only a thin film of developer solution is applied to each plate, any variation of any part of the surface of the plate from being substantially flat and horizontal and any variation in the thickness of the film of developer and any variation in the dwell time of the developer on different areas of the plate can result in the improper development of the coating.
The use of a wire-wound rod as a coating means is well suited to the continuous coating of web materials with a fluid, and well known in the art. Typically, a wire-wound coating rod is used in a coating method where some volume of fluid is continuously applied to the web surface prior to the rod, and the rod serves to meter the amount of fluid allowed to remain on the web surface. However, the coating of individual, discrete plates requires the ability to precisely initiate the coating process and precisely terminate the coating process on individual plates delivered at irregular intervals. In the case of lithographic printing plates, the developer fluid must be applied in the correct amount uniformly distributed across the width and length of the plate, with minimal waste.
The use of a wire-wound rod in metering the developer in a lithographic plate processor is known in the art. U.S. Pat. No. 4,737,810 teaches the application of excess developer with the wire-wound rod serving as the means to meter off the excess into some recovery means. The rod thus serves as the means to control the volume of fluid consumed in the development process. The developer fluid is applied to the plate ahead of the wire-wound rod and it is indicated in this patent that the path between the delivery of the fluid and the metering at the wire-wound rod is sufficiently short that development does not commence within this area. The excess developer removed in this area is intended to be reused.
Typical imaging methods include exposure to radiation and writing by ink jet. As is well known in the art, the imaging process renders the coating soluble in the imaged areas of a positive-working plate and renders the coating insoluble in the imaged areas of a negative-working plate. In either case, it is the coating which has been rendered soluble or the coating which has remained soluble that is removed. The particular compositions of the developer solutions for these different types of printing plates are well known. For example, many of the printing plates currently in use are positive-working plates and have coatings that contain alkali-soluble resins, specifically phenolic or acrylic resins. These coatings usually contain dissolution inhibitors that render them insoluble in the alkaline developers. The imaging process reverses this dissolution inhibition and the coating then becomes soluble in the areas subjected to the imaging radiation.
Excess developer that has been applied to the surface, metered off by a wire-wound rod, and subsequently recovered has been exposed to the atmosphere, and as such is subject to degradation. It is well documented in the art that atmospheric carbon dioxide rapidly reduces the alkalinity of aqueous alkaline developers of the type very commonly used in the processing of positive-working plates. Thus the recovered developer that is being reused will not have the same alkalinity as fresh, new developer. This recovered developer is in fact therefore reused in a way that with each application of developer to the plate, some fraction of the developer metered off by the wire-wound coating rod will have been removed in a previous cycle of development. Thus the repeated exposure to carbon dioxide and resultant degradation will further alter the effective alkalinity of the developer. Further, if the developer is applied some distance ahead of a wire-wound rod, some degree of development is certain to take place and it may very well be uneven.
With positive-working plates, the difference in the solubility of the imaged and non-imaged areas of the coating is generally less than the difference in solubility for negative-working plates. For that reason, the development process is more critical for positive-working plates. Also, the development mechanism for positive-working plates is a percolation process and a quiescent film of developer solution is critical. Any relative movement between the developer and the surface of the plate must be minimized or eliminated. Furthermore, the film of developer must be uniform with no bubbles. For these reasons, it is critical how the developer is applied to the plate.
The present invention is directed to an apparatus and a method for applying a uniform film of a fluid to the surface of a flat workpiece as it is being conveyed through a workstation. The invention is specifically directed to the development of lithographic printing plates and comprises a novel system and method for applying the thin film of developer solution to each plate in a controlled manner using a wire-wound rod coating device and a unique manner of metering and feeding fresh fluid to the wire-wound rod. Specifically, the fluid is gently fed onto the wire-wound rod by simple volumetric displacement and overflow and the flow is controlled by sensing the beginning and end of each workpiece or plate. The developer is allowed to dwell on the plate as it is conveyed across a platen or other support structure for a sufficient time to allow for percolation into and/or dissolution of the soluble areas of the coating.